On Indian summer monsoon droughts and teleconnections over the past millennium

Historical documentary accounts from the Indian subcontinent document several decade-to-multidecade clusters of severe Indian summer monsoon (ISM) droughts over the past millennium. Many of these putative droughts have no counterparts in the instrumental period. An objective assessment of the severity and frequency of these droughts and their teleconnections to other parts of the climate system remains uncertain. Here, we use the Paleo Hydrodynamics Data Assimilation (PHYDA) product to address this gap. PHYDA reproduces historical intervals of increased drought frequency with high fidelity and reveals multiple instances of multi-year “black swan” droughts — rare, severe events with significant societal impacts that are consistent with historical accounts. We find that droughts, as well as extended periods of dryness, occurred under both El Niño and non-El Niño conditions, with El Niño explaining ~ 49% of all drought instances. A large number (~42%) of non-El Niño type droughts were forced by cooler extratropical SST anomalies in the North Atlantic region. While El Niño was an important driver, its association with droughts varied considerably, accounting for between 15-80% of droughts depending on the century. The PHYDA’s millennial-length perspective supports expanding the El Niño-centric paradigm of droughts into a framework that includes extratropical teleconnections.